There are many situations where it is desirable to be able to monitor the pH and/or oxygen concentration of an aqueous medium. For example, in power generating facilities employing aqueous cooling systems, corrosion continues to be a problem. Corrosion refers to the loss or conversion into another insoluble compound of a solid in contact with a liquid, e.g. pipes in contact with water of a coolant system. Corrosion can result in significant economic costs. For example, corrosion can weaken the physical strength and integrity of pipes, thereby necessitating their replacement. Furthermore, corrosion can pose significant safety risks, as corroded material is more likely to break. Two factors that contribute to the corrosion potential of an aqueous medium are the oxygen concentration and pH of the medium.
In order to counteract the effects of corrosion, sensors have been developed which can monitor the oxygen concentration and pH of the system. If the value of either the oxygen concentration or pH varies from a value associated with a low corrosion potential, measures can be taken to return the oxygen concentration or pH to a more desirable value, e.g. by addition of NaOH if the pH becomes too acidic.
Typically the oxygen concentration and pH of water in a cooling system have been monitored by taking a sample of water from the system in what is known as a "grab sample basis." However, in this type of monitoring, the sample tested is not at the same temperature as the sample in the system. Because the emf of an electrochemical sensor is affected by the temperature of the medium being tested, in such methods one does not obtain an emf from which the true pH or oxygen concentration of the medium can be derived.
To address the problems of monitoring on a "grab sample basis," sensors have been developed for monitoring the oxygen concentration and pH of an aqueous medium in situ. For temperatures exceeding 350.degree. C., yttria or calcia stabilized zirconia solid-state electrolyte sensors have found use. However, these sensors are not suitable for use in temperatures below 350.degree. C. because of the poor conductivity of the electrolyte at these lower temperatures. Likewise, glass electrodes have been found suitable for use with aqueous mediums at ambient temperature. However, these sensors fail at higher temperatures due to degradation of the glass membranes. Thus, the opportunity to monitor the oxygen concentration and pH of an aqueous medium in situ at temperatures between about 25.degree. and 350.degree. C. has remained limited, if not non-existent.
Accordingly, there is continued interest in the development of sensors which can be used in situ to monitor the oxygen concentration and pH in aqueous mediums with temperatures ranging from 25.degree. to 350.degree. C. Such sensors should be physically and chemically inert in the working environment of the coolant system in which they are employed, e.g. the temperature and pressure range of the aqueous medium to be tested. Furthermore, such sensors should be able to provide an accurate reading in a relatively short period of time, so that deviations in the corrosion potential of the aqueous medium can be quickly remedied.